An electric visit to coal-fired Didcot

On Thursday, 18 June 2009, a minibus awaited attendees at the gate of Didcot power station. It chauffeured them to the main reception area for a highly informative introduction to the site and its importance in the wider context of electricity production in the country.

Operated by RWE npower, a leading energy provider with an extensive portfolio of energy generation, the site houses two independent facilities. Didcot A is a near 40-year-old coal and gas-fired facility, with an efficiency of 35% and has six water-cooling towers producing 2GW of power. Didcot B uses a state-of-the-art combined cycle, gas turbine with an efficiency of 55% and contributes another 1.36GW to the grid at peak times. Both facilities combined are able to generate power enough to supply three large counties (about three million people).

Most of the electricity produced is used to balance electricity consumption in the southeast of the country. Didcot A has a lifetime of a few thousand operating hours left, or will be taken out of commission in 2015, whichever is earlier (Didcot A has opted out of the Large Combustion Plants Directive). In order to preserve its lifetime, Didcot A mainly operates during peak times and especially in the winter. A cold start takes three days to fire it up to full power. Didcot A consumes about 20,000 tonnes of coal per day at full capacity – that's about 10 huge trains worth.

Valerie, a very knowledgeable and friendly tour guide, took the group on a tour of Didcot A, which, at the time, was undergoing maintenance and therefore fully switched off. The circuit began with a drive to the coal storage facility, where a mountain of just under one million tons of coal (enough for a few BBQs on a hot summers evening) was waiting to be turned into electrical energy. The coal was shipped to Bristol from overseas (mainly Russia and Australia) owing to its low sulphur content. Giant conveyer belts move the coal into the main building, where huge mills pulverise the material before it is sent to the boiler's burner.

Didcot A has four independently operated 500MW generating units. The steam's kinetic energy is transferred to one giant high pressure turbine, an intermediate pressure turbine and three low pressure turbines. The station can be oil-fired if necessary, albeit with a drop in yield.

Another type of fuel used in small quantities is biomass, otherwise known as ‘carbon-neutral' fuel. It is combined with coal before it reaches the boiler. Biomass consists mainly of wood chips or sawdust and palm kernel expeller. Any ash left over from the combustion process is sold as additives for thermal insulation or cement. The scale of the site and of each of its components made the visit even more memorable. This was a very successful outing and thanks must go to Dr Alan Baylis of the SCI BioResources Group for organising the trip for both Groups.

Dr Oliver Choroba
Thames and Kennet Group

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